Altimeter shift during night - suggested solution

Something similar here, lots of pressure changes cause of the storm resulting in a wrong altitude, but my watch corrects this at night. Even while I don't have my phone connected during sleep. Mine got me from - something it got during the day back to a more realistic value at 03:00 in the night. No idea where it comes from, maybe last known location height from the internal map.

My shift was exactly 5 minutes before set alarm. At 5:40. The whole night it was OK. 

Now I want to look at what time my sleep ended, but it's missing even if I looked at it in the morning! So next big bug. ️

I cant understand why people get so anal about the altimeter showing an "incorrect" reading after several hours.

There isn't actually any altimeter sensor in the watch at all. Altitude is derived from pressure changes on the barometer. Pressure goes down, altimeter reading goes up, pressure goes up altimeter reading goes down.

There are 3 basic ways of calibrating the altimeter. DEM, GPS, Known point.

Once calibrated if the pressure changes for any reason your altitude will change unless you put the watch into barometer mode.

So just get on with your life.

When you want to start an activity just calibrate the altimeter using 1 of the 3 methods above but bear in mind if the pressure changes so will the altitude assuming the watch sensor is in altitude mode.

If your activity is over several hours or somewhere with changing weather conditions then calibrate the altimeter several times.

Pilots use a barometric altimeter and as they fly around they recalibrate it, sometimes every 10 - 15 minutes, its just what you have to do with a barometric altimeter.

About known point, does that save in the watch? 
Ie, if i have a few points, 5krunner wrote about that recently, will the wach use them again when im there, or is it a one off?

Calibrate it using a base topographical map. In the UK I find the OS Maps and OS Locate apps very useful for this.

I wish there was a feature of the watch to automatically calibrate the altimeter periodically using a known elevation of your house. I know my house is at 10m above sea level so I have to calibrate it manually all the time because of pressure drift. This is particularly relevant to the UK which experiences very changeable, stormy weather in the winter months.

I remember hearing something about saving your home as a Saved Location with the correct elevation but I’m not sure how this works in practice.

I totally understand how a barometer works and why the fenix is prone to error with every weather/pressure/altitude change. That's why I put it in barometer mode when at home. The thing that is annoying, but probably very difficult for the watch as well, is to know the difference between actual altitude change or just pressure change. I've noticed over the last week that the barometer was off the whole time by a few mb's while on Auto mode. It translated the pressure change to altitude change. But as soon as you go to calibrate, it'll show you the correct sealevel pressure. So somewhere in the watch it is always correct, but knowing what causes the change in pressure is difficult for the watch. Probably something we'll have to live with until they find a smart way to overcome this. 

2572376 said:

There isn't actually any altimeter sensor in the watch at all. Altitude is derived from pressure changes on the barometer. Pressure goes down, altimeter reading goes up, pressure goes up altimeter reading goes down.

With Altimeter sensor mode=Auto, then watch suppose to guess if you are moving or not. When moving alti is changed and when not, baro is changed according to pressure changes.

What you describe is when watch is guessing you are moving and then elevation i 'unlocked'.

My conclusion is that what OP describe is problem with auto, not that op doesn’t understand how the alti is working in watch.

its exactly what you said, A guess. It tries to give a reasonable answer, but its an educated guess. Think about this, they have been doing barometric altimeters in watches for years, if it was that simple it would always be right, just like getting the time right is a given for watches today.

In auto mode it makes a best guess as to if its you ascending / descending or if the pressure is changing, it uses GPS etc to see if you are moving and the result is a GUESS, not a definitive answer.

The best way to get the altimeter right is to put it into altimeter mode and calibrate at the start of an activity. If your activity is on flat ground put it into barometer mode, if the activity includes altitude changes put it into altimeter mode and recalibrate every time you think the pressure might have changed because of the weather.

So keep an eye on the weather and see what its doing.

Temperature changes the pressure

Humidity changes the pressure

Local magnetic conditions also change the pressure but you are unlikely to ever need to worry about that one.

I asked in another post if the watch use the barometric pressure to calculate the altitude during activity or it use the GPS or DEM data. As per your comment, I think the watch is working fine and what happened the other day it was due to a weather pressure changing quickly. 

For me the new altimeter calculation is working better . I think is more sensitive and I think this is the reason why it drifts more with the weather. As you, when there is bad weather and I'm at home, I keep the watch in Barometer Mode.